Revisiting solute-based grain refinement in pure and recycled Al alloys: Mechanistic insights into Al3(Sc,Ti) formation and recycling-resilient nucleation

Solute-based grain refinement elements (i.e. Ti, Zr) are not widely commercialised compared with external particle-based approaches such as TiB 2 in Al–Ti–B master alloys, due to their limitation in low effectiveness under low concentration, sensitivity to composition and cost–performance. However, with the increasing demand for sustainable aluminium alloys, limitations of conventional particle-based, and once-off used grain refiners are becoming more apparent, including the accumulation of non-metallic inclusions (TiB 2 ), and related solutes accumulation (i.e. Ti, B) during repeated grain refiner additions. Revisiting and developing sustainable grain refinement strategies has therefore become essential under Net-Zero manufacturing requirement. This study systematically reassesses the grain refinement behaviour of Sc, Zr, and Ti in pure and recycled Al alloys, aiming to establish a mechanistic understanding linking solute effects, intermetallic formation, and oxide-mediated heterogeneous nucleation. This study reveals a new route to achieve a combination of sustainable, solute-based grain refinement with high nucleation efficiency and exceptional stability under repeated recycling, enabled by Sc. It is shown that in situ formation of Al 3 (Sc,Ti) intermetallic compounds produces fine, uniformly distributed particles with near-zero lattice misfit with α-Al. These particles act as highly efficient and potentially recyclable nucleation sites, as they can fully dissolve and re-form during repeated remelting and solidification cycles. In particular, Al 3 (Sc,Ti) exhibits exceptional stability and nucleation potency, retaining its effectiveness despite thermal exposure and oxidation associated with recycling. This behaviour suggests a new pathway toward grain refiners that are not only highly effective but also more sustainable and recycling-compatible than conventional TiB 2 -based systems.